As is well known, prospecting for minerals of commercial or other value (including but not limited to hydrocarbons in liquid or gaseous form; water e.g. in aquifers; and various solids used e.g. as fuels, ores or in manufacturing) is economically an extremely important activity. For various reasons those wishing to extract such minerals from below the surface of the ground or the floor of an ocean need to acquire as much information as possible about both the potential commercial worth of the minerals in a geological formation and also any difficulties that may arise in the extraction of the minerals to surface locations at which they may be used.
For this reason over many decades techniques of logging of subterranean formations have developed for the purpose of establishing, with as much accuracy as possible, information as outlined above both before mineral extraction activities commence and also, increasingly frequently, while they are taking place.
Broadly stated, logging involves inserting a logging tool including a section sometimes called a “sonde” into a borehole or other feature penetrating a formation under investigation; and using the sonde to energise the material of the rock, etc, surrounding the borehole in some way. The sonde or another tool associated with it that is capable of detecting energy is intended then to receive emitted energy that has passed through the various components in the rock before being recorded by the logging tool.
Such passage of the energy alters its character. Knowledge of the attributes of the emitted energy and that detected after passage through the rock may reveal considerable information about the chemistry, concentration, quantity and a host of other characteristics of minerals in the vicinity of the borehole, as well as geological aspects that influence the ease with which the target mineral material may be extracted to a surface location.
The boreholes used for the purpose explained above may extend for several thousands or tens of thousands of feet from a surface location. This makes it hard to communicate with a logging tool that is conveyed a significant distance along the borehole.
It is known to provide logging tools that are essentially autonomous in use. Such tools may include energy sources such as electrical batteries, together with one or more on-board memory devices.
An autonomous tool of this kind may be conveyed e.g. by inserting it supported on drillpipe, or pumping using any of a variety of fluids, to a great depth in a borehole where it may perform logging activities as outlined above. Since the tool is self-powered it may carry out logging operations following deployment, and may record log data using the on-board memory.
The tool is recovered to a surface location at the completion of logging activity. At this point the log data may be downloaded from the memory, processed, analysed and/or displayed in a variety of ways that will be known to the worker of skill in the art.
An autonomous logging tool, however, is not normally capable of signalling correct deployment at its downhole location; nor is it usually capable of sending log data to a surface location in real-time; nor may it normally receive complex control commands from a surface location. Of particular significance in some situations is the fact that the data cannot be accessed until the tool is retrieved completely to a surface location.
Some techniques for signalling between autonomous tools in downhole locations and surface equipment are known. These generally involve the generation of coded pulses in the fluids (that may be “muds” of a kind familiar to those of skill in the art, or other fluids) that fill the borehole and that are used inter alia for pumping tools between surface and downhole locations.
These mud pulses, however, amount to very narrow bandwidth, low bit-rate communications that are not at all suitable for conveying log data in real-time. Moreover the mud pulses require energy to generate and can be ambiguous due to their propagation over many thousands of feet of the borehole depth. Mud pulse signalling therefore is often of little help in the controlling of logging tools and the rapid acquisition of data.
One known logging technique, sometimes referred to as “logging while drilling” (LWD), involves logging a hydrocarbon reservoir while drilling it to create a producible hydrocarbon well. LWD requires the incorporation of an operative logging tool in a mineral drill, or at any rate the positioning of the logging tool in close proximity to the tool, and is an example of the general requirement in logging, indicated above, for log data to be acquired while extraction work is taking place. As drilling a borehole takes significant time, typically days, slow data rates although a disadvantage are useable in this application. Other logging techniques, to which the invention additionally pertains, would preferentially benefit from the rapid acquisition of log data. Examples of such techniques include memory logging with Wireline tools using techniques known as “drop-off”, “pump down” and “Shuttle deployment.”
It has for some decades been known to communicate with logging tools using so-called “wireline.” A wireline, as is well known in the art, is an armoured cable that may be used for the purposes of supporting a logging tool while it is being withdrawn upwardly along a borehole or well during logging; transmitting, using electrical/electronic signals, data from the logging tool rapidly to a surface location; and transmitting control commands for the logging tool and in some cases power for powering the operations of the tool from the surface location.
Wireline logging techniques have proved extremely useful over many years. In particular wireline avoids many of the speed and bandwidth problems of slower communication techniques such as mud pulse signalling, thereby making wireline-supported logging tools more attractive than autonomous tools in various situations.
However, one difficulty associated with wireline logging tools is that it is not generally possible to maintain a connection during e.g. an LWD operation since the wireline presents an obstacle to jointing of the drill pipe at the surface. It therefore is often required to make and break electrical connections in downhole locations in order to permit the selective use of wireline and thereby avoid wireline fouling problems as would arise if the logging tool remained connected to the wireline during an LWD or other, similar operation.
This is also of particular importance during for example the deployment of a logging tool that is conveyed to a downhole location within or through drillpipe. Gathering data from the tool under such circumstances necessarily requires the movement of drill pipe. Such movement often creates a requirement for selective power and/or communications connection of the tool to and its disconnection from other components in the logging toolstring and/or to wireline.
The downhole environment is usually extremely harsh, partly because of significant fluid pressures that exist and also because various chemicals present in boreholes are not compatible with the use of electrical signals for data and power transmission. This could be because the chemicals are for example chemically aggressive and thereby degrade connector terminals, or because they are electrically conducting or insulating in ways that can interfere with the performance of electrical and electronic equipment exposed to them.
The damaging physical conditions in a downhole location make it extremely hard to design a reliable, releasable connector that meets the multiple requirements set out above.
Conventional plug-and-socket electrical connectors are available for use in downhole environments, for example in order to connect wireline to a logging toolstring. These connectors, however, require assembly at a surface location before being conveyed downhole in a borehole. Many such connection designs cannot be “made” after being “broken” in a downhole situation as may occur when the wireline is pulled away from the toolstring.
One type of connector that has been proposed as a solution to this difficulty is a so-called “wet connect” or “wet connector”. This type of connector is intended for repeated making and breaking of electrical connections in remote environments in which there are fluids such as borehole fluids.
A typical wet connector is constituted by a pair of rigid jack-type connector elements a female one of which has an elongate, open-ended, circular-section cavity for receiving a cylindrical male connector. Electrical terminals formed in the interior of the female element and on the male element create an electrical connection when the male element is inserted correctly into the female.
Wet connectors, however, suffer from numerous problems one of which is that if any borehole fluid becomes interposed between the terminals respectively of the male and female elements, undesirable short circuits, open circuits and other anomalies, depending on the character of the borehole fluid, may arise.
Certain wet connector designs include features the aim of which is to minimise the chance of borehole fluid ingress in this way, but these features often are not successful. As a result for example, the anti-ingress features may make it less likely on mating of the male and female connector elements that the terminals will contact one another in a satisfactory manner.
Moreover, borehole fluids as indicated may be chemically aggressive, abrasive and/or under very high pressure. These factors tend to make the anti-ingress features of the wet connectors fail prematurely.
Yet another problem associated with wet connectors is that they tend to occupy a large volume in the vicinity of the toolstring parts requiring connection. This makes them unsuited for use in conjunction with mechanical latch arms of the kind that are often used for the temporary securing of parts of a toolstring, such as relatively uphole and downhole elements of a sonde assembly, together. This is particularly relevant when the maximum tool diameter is a constraint, i.e. when passing through 3.5″ drillpipe that is common in the industry.
Thus, there is a need for a data and/or power transmitting arrangement that avoids or at least ameliorates one or more drawbacks, of the prior art, such as those indicated above. It would be particularly desirable to provide a coupling arrangement that is reliable in downhole LWD situations, as well as at other times, while being reuseable multiple times.